The present invention relates generally to valves for high pressure hydraulic systems and in particular to fast-acting solenoid valves for use with hydraulically actuated control elements for a steam turbine.
The final control element in a steam turbine control system is generally a plurality of very large, hydraulically positioned valves through which steam flows into the turbine. For example, the steam admission control valves are preferably positioned hydraulically in accordance with a control signal which directs a servo valve to increase or decrease hydraulic pressure on the valve control mechanism. In addition to the normal valve positioning means, it is usual and prudent to also provide auxiliary valving in the hydraulic system so that the steam valves can be very rapidly closed by a trip signal in the event there is loss of load or some similar malfunction which demands a rapid shutoff of the steam flow to the turbine. Rapid closure of the steam valves is carried out in these situations by providing for a quick dump of the hydraulic fluid which sustains the steam valve position. A fast acting solenoid valve is incorporated in the trip system to facilitate the tripping action and is used to initiate the turbine trip in certain instances. It is apparent that the fast acting solenoid valve must perform reliably if the turbine is to be protected in those situations requiring a trip of the steam valves.
In the past, fast acting solenoid valves for performing this particular turbine control function have suffered from a number of drawbacks which have detracted from their reliability. Chief among these are failures to respond to a trip signal because of jammed or stuck parts and failure to pick up under marginally low voltage conditions. In addition, certain fast acting solenoid valves have created pressure transients which are reflected back into the hydraulic system during operation and which have been of sufficient magnitude to disturb other components of the hydraulic system. In general it has also been necessary to protect these valves from the effects of contaminants which might otherwise buildup in the poppet area of the valve.
Furthermore, such sealing of the poppet allows for relatively large clearances, between the poppet and the bore minimizing the chance of jamming due to misalignment or sticking. The surface area exposed to hydraulic pressure at the end of the poppet at the switched actuation chamber end is greater than the corresponding area at the opposing end with the poppet in the normal position, so that, with equal pressure in the chambers, the hydraulic forces are unbalanced and the poppet is held firmly in the first valving position.